1. Field of the Invention
This invention relates to a relief pattern producing method and a relief pattern sheet produced using such the method.
2. Description of Related Art
Methods for producing a relief pattern sheet have previously been proposed. According to a method disclosed, for example, in U.S. Pat. No. 4,268,615, a layer of a desired pattern is formed on the surface of a thermal expansile sheet, wherein the pattern layer is made of a material being more optically absorptive than the thermal expansile sheet, and wherein when the surface of the thermal expansile sheet is exposed to light, the patterned portion of the sheet is selectively heated to rise by virtue of a difference in optical absorption.
Moreover, Japanese Laid-Open Publication No. 61-72589 discloses a pattern forming method, wherein a highly optically absorptive pattern is formed by thermal transfer, and this pattern is exposed to light to produce a relief pattern corresponding to an image signal on an expandable foaming substance.
These methods permit a relief pattern to be formed on a sheet with a simple operation.
However, when a plurality of figures are formed on a thermal expansile sheet utilizing these methods and the sheet is then exposed to light, thermal interaction arises between the figures, which makes it impossible to create a uniform relief pattern.
Specifically, where an individual figure produces heat, resultant heat dissipates to surrounding low temperature areas because of the lack of another heat generating figure around that figure.
On the other hand, where a plurality of adjoining figures simultaneously produce heat, an ambient temperature around the figures rises, which in turn delays dissipation of heat resulting from optical absorption. For this reason, a thermal expansile layer must be heated for a long time, and therefore, the degree of expansion of the figures becomes larger when compared with the case of the independent figure.
In addition, where figures adjoin only in one direction, the speed of dissipation of developing heat is delayed only in this direction, and hence, a part of the figure being adjacent another figure is heated much more. Accordingly, only this adjoining portion expands significantly, resulting in distorted expanded figures.
The above mentioned phenomena will be explained with reference to following examples.
FIGS. 2A and 2B are top and cross-sectional views respectively of a relief pattern sheet after a circle 15, having a diameter R1 and being formed on a non-illustrated thermal expansile sheet by thermal transfer, has thermally expanded upon exposure to light. D1 designates the height of a raised part.
FIGS. 3A and 3B are top and cross-sectional views respectively of a relief pattern sheet after four circles 16, 17, 18, and 19, each having the same diameter R1 as that of the circle 15 shown in FIG. 2A and being formed at intervals of L2 by thermal transfer, have thermally expanded upon exposure to light. In these drawings, the relationship between the diameter R1 of the circle and the interval L2 between the circles will be written as L2=0.2.times.R1.
Upon exposure of the thermal expansile sheet on which a plurality of circles, each circle having the same area, are formed at small intervals to light, each circular region absorbs an equal amount of light to produce heat. Heat developing from four circular regions is substantially the same, and the heat simultaneously dissipates to surrounding areas of the circular regions in the thermal expansile sheet.
First, consider the dissipation of heat from the circle 17. The circle 17 is sandwiched between the circles 16 and 18. Heat flows from two circles into regions sandwiched between the circles 17 and 16 and between the circles 17 and 18, whereby the temperatures of these regions increase. Generally, the speed of dissipation of heat is proportional to a temperature gradient in the direction of dissipation, and therefore, dissipation of heat from the circle 17 is delayed when a temperature in the direction of dissipation of heat has risen more rapidly in this case. This causes the sandwiched regions to be heated for a longer time, and the regions expand much more when compared with the independent circle shown in FIGS. 2A and 2B. For this reason, as shown in the cross-sectional view of FIGS. 3A and 3B, the height D2 of the raised portions is larger than D1 of FIG. 2B. The circle 18 is also sandwiched between the two circles 17 and 19, and therefore, the circle 18 expands in the same manner as the circle 17.
Since the circle 17 is formed on the right of the circle 16, heat flows from two circles into the region sandwiched between the circles 16 and 17 in the same manner as previously mentioned, so that the temperature of that region resultantly increases. Therefore, the speed of dissipation of heat from the circle 16 to the right becomes equivalent to that of the circle 17. On the other hand, no circle is adjacent the left of the circle 16, and hence, heat is given off from the circle 16 to the left in the same manner as the dissipation of heat from the circle shown in FIGS. 2A and 2B.
Consequently, heat is radiated from the circle 16 slowly toward the right but rapidly toward the left. This results in figures disproportionately expanding in a lateral direction. The height of a right half of the raised portion of the circle 16 becomes substantially equal to that of the circles 17 and 18, but the height of a left half of the raised portion of the circle 16 becomes substantially equal to that of the independent circle shown in FIGS. 2A and 2B. The circle 19 is a mirror image of the circle 16, and therefore, the height of a right half thereof is lower, but the height of a left half of the same is higher.
In this way, when a plurality of relief patterns are formed on one sheet, if figures are too closely spaced from each other, dissipation of heat from the figures, whose temperatures are increased after being exposed to light, will interact with dissipation of heat from surrounding other figures, resulting in raised figures having non-uniform shapes and sizes.